This Phase I study demonstrated the basic feasibility of developing a hybrid active/passive noise canceling stethoscope for rotorcraft aeromedical evacuation use. The hybrid design makes full use of active and passive noise reduction techniques, and incorporates: (1) primary and reference stethoscopes to pickup the relevant patient sounds and to measure the surrounding ambient noise environment; (2) a hybrid noise canceling headset used by the medic to listen to the transduced heart/lung sounds and to reduce the ambient noise levels; and (3) a custom ANC processor to further reduce noise pickup at the primary stethoscope. Under the Phase I effort we reviewed commercially-available hardware, assembled candidate components in a prototype system, developed custom ANC algorithms for patient signal processing, and demonstrated end-to-end operation of the system. Engineering evaluations of noise reduction capability and psychoacoustic evaluations of patient sound clarity were made to demonstrate system feasibility and to identify system requirements for full-scope prototype development under a follow-on program. The Phase I evaluation clearly demonstrated the system's ability to extract clean patient sounds in high ambient level noise, in situations in which one would normally experience inaudible patient sounds using conventional or electronic stethoscopes. Stethoscope, Active Noise Cancellation, Medevac, Noise, Auscultation Device, SBIR, Phase I, RAD II.